The IGBT is a high power switch having an insulated gate for controlling the conductivity states of a bipolar transistor. The IGBT has diverse applications for switching high amperage currents flowing between a power supply and an electric load. The Assignee of the present invention has developed VSCF (variable speed constant frequency) electric power generating systems for use in airframes which generate three phase 400 Hz. current with the use of a three phase inverter having IGBT power switches. The IGBTs in a VSCF electric power generating system are configured in a three phase inverter which are switched between on and off states to generate three phase displaced squarewaves occurring at the fundamental frequency of 400 Hz. which are phase displaced by 120.degree.. The DC electric power for generating the inverter is produced by three phase rectification of a three phase alternator which is driven from a power takeoff from a propulsion engine of the airframe or a power unit such as an APU.
It is highly desirable that the three phase inverter utilizing IGBTs in a VSCF electric power generating system have the highest possible current switching power rating and the highest possible reliability. Current designs of IGBTs are limited by the temperature of operation. An IGBT assembly in a three phase inverter used in a VSCF system must be able to withstand repeated thermal cycling which stresses the assembly with a high rate of thermal expansion caused by the repeated heating and cooling of the assembly during operation.
Current designs of IGBTs utilized in three phase inverters in VSCF systems dissipate the heat of switching through the bottom of the case of the IGBT assembly and semiconductor die through a heat radiator which may be a heat sink such as a copper molybdenum base plate. The base plate is thermally coupled to a beryllium oxide substrate which has a high thermal conductivity and which is electrically insulative on which IGBT semiconductor die is mounted. The power rating of the IGBT is limited by the thermal conductivity between the IGBT die and the beryllium oxide substrate and copper molybdenum base. The current designs of IGBTs are limited in power by not dissipating substantial heat through the top surface of the assembly.
Compression bonded bipolar power transistor assemblies utilize heat radiators which radiate heat from the top and bottom of the assembly.
Current IGBTs rely upon wire bonds between electrode terminals and the electrodes of the IGBT and/or metallization between the electrode terminal and the electrode which is disposed on the surface of the beryllium oxide. In current designs of the IGBT, such as those manufactured by Advanced Power Technology, a wire bond is utilized between a gate terminal and the gate electrode of the IGBT die. Current designs of IGBTs also use metallization disposed on the surface of the beryllium oxide substrate between the collector electrode of the semiconductor die and a collector terminal which is soldered to the beryllium oxide substrate at the point of attachment to metallization running to the collector electrode. Current designs of IGBTs also electrically connect the emitter terminal to the emitter electrode with wire bonds.
The use of wire bonds in IGBTs creates reliability problems for high power applications when large currents flow through the wire bonds between external assembly terminals and internal power electrodes such as the emitter. The use of a wire bond to complete the electrical connection between the emitter terminal of a IGBT housing and the emitter electrode creates reliability problems which are caused by excessive heat or strain produced by high current flow. Excessive emitter current can result in the wire bond fusing which creates a high resistance contact. Additionally, the repeated thermal cycling caused by the turning on and off of IGBTs in a three phase inverter such as in a VSCF electric power generating system produces severe thermal stress which can cause a strain induced failure of the wire bond.
U.S. Pat. No. 4,677,741 discloses a method of manufacturing a package for a high power integrated circuit. The '741 patent discloses the use of wire bonds to connect external electrodes to power electrodes of the semiconductor chip.
U.S. Pat. No. 4,748,103 which is assigned to Advanced Power Technology, discloses a IGBT of the type which has been utilized by the Assignee in VSCF electric power generating systems.